Methods for producing a product ester by the esterification of a lower hydrocarbyl carboxylic acid with an alcohol in the presence of an esterification catalyst are well known in the art. A preferred conventional process for the manufacture of such product esters comprises esterifying the carboxylic acid and alcohol in a reactor containing a substantially anyhydrous reaction medium which also contains the esterification catalyst; obtaining a volatilized product ester-water azeotrope from a distillation column connected to the reaction vessel, and recovering the product ester via phase separation of the product ester-water azeotrope in a separate vessel. For instance, British Patents 1,173,089 and 1,262,645 disclose such a process using sulfuric acid or para-toluene sulfonic acid as the esterification catalyst. Moreover, if desired, a dialkyl sulfate such as dimethyl sulfate or diethyl sulfate may be employed as a catalyst precursor to form an in situ hydrolyzed catalytic equilibrium mixture of monoalkyl sulfate and sulfuric acid. Further European Patent Publication No. 009886 also teaches the use of a strong mineral acid such as sulfuric acid or p-toluene sulfonic acid as the catalyst in such a process, while European Patent Publication No. 158,499 advocates that the catalyst be an alkyl sulfonic acid of the formula RSO.sub.3 H wherein R is a C.sub.1 to C.sub.12 substituted or unsubstituted aliphatic hydrocarbyl group with the added proviso that the alkyl sulfonic acid have a desulfonation temperature in excess of 186.degree. C., the preferred catalyst being methanesulfonic acid (CH.sub.3 SO.sub.3 H).
However, it has been found that the presence of acidic sulfate (SO.sub.4.sup.-2) in the product ester can be highly detrimental to the storage capabilities of such product esters. For instance, the presence of even a fairly moderate amount of acidic sulfate in the product ester has been surprisingly found to be the cause of product ester instability with regard to unacceptable carboxylic acid formation via decomposition of the product ester upon storage. For example a product butyl acetate containing about 0.70 ppm SO.sub.4.sup.-2 and having an initial carboxylic acidity of about 0.010 percent acetic acid at 24.degree. C., was found to have increased in said acidity to 0.05 percent after days of storage; to 0.125 percent after 43 days and to 0.175 percent after 69 days. Such high amounts of acetic acid obtained upon storage would render the stored product ester unacceptable to the customer.
The acidic sulfate found in the product ester is believed to catalyze hydrolysis of the product ester back to the carboxylic acid and alcohol starting materials thus limiting or shortening the shelf life or storage capability of the product ester. It is further believed that the source of such acidic sulfate in the product esters is derived from thermal decomposition of the corresponding in situ formed sulfonate esters of the acid catalyst employed during the esterification process to sulfur oxides (e.g. SO.sub.2 /SO.sub.3) which appear in the product as acidic sulfate (i.e., SO.sub.4.sup.-2). The sulfonate esters are derived from the esterification of the acid catalysts themselves with the alcohol present in the reaction medium. Thus the production of product esters containing as little of such acidic sulfate as possible is very desirable to the product ester manufacturer.